I admit not having worked extensively with XULRunner. Maybe 4 years ago was it, but I know that html can be used via the html namespace https://developer.mozilla.org/en/XUL_Tutorial/Adding_HTML_El...

It looks quirky, but I thought the most important part of using web technologies is to use Javascript and CSS; which can be applied over XUL elements just like HTML.

Also compared to the current project it is vastly superior in terms of features and functionality that can be tapped into with Javascript.

There is space for improvement, like with most software, but it is in fact a tested technology already.